Field of the Invention
The present invention relates to an image forming apparatus including an optical scanning device.
Description of the Related Art
In an image forming apparatus of an electrophotographic type such as a digital copying machine, a scheme is known in which a charged photosensitive member is exposed by an optical scanning device to form an electrostatic latent image, and an image is formed through subsequent steps of development, transfer, and fixing. The optical scanning device includes a semiconductor laser configured to be turned on according to an image signal, a rotary polygon mirror configured to perform a scan with a laser beam from the semiconductor laser turned on, and a lens configured to condense the laser beam on the photosensitive member. The optical scanning device includes an optical sensor for sensing a position of the laser beam used for the scanning (Beam Detector; hereafter, referred to as BD). Upon receiving the laser beam, the BD generates a light reception signal. Based on a generation timing of the light reception signal (hereafter, referred to as a BD signal) (detection timing of the laser beam), a writing start position of an image in a scanning direction of the laser beam is controlled, and a rotational speed of the rotary polygon mirror is controlled. When a user gives instructions of image formation start, the rotary polygon mirror starts rotating, then the rotational speed of the rotary polygon mirror reaches a predetermined speed. After the rotational speed of the rotary polygon mirror has reached the predetermined speed, the semiconductor laser starts light emission operation (hereafter, referred to as BD search), a timing of scanning with the laser beam on a light receiving surface of the BD is acquired within one scanning period of the laser beam.
An image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2015-49481 starts up a rotary polygon mirror based on a period of an FG signal which is generated according to a magnetic pole pattern of a permanent magnet in a motor. The image forming apparatus causes a semiconductor laser to be turned on continuously, or intermittently with a high period in response to a rotational speed of the rotary polygon mirror reaching a predetermined speed. By the image forming apparatus causing the semiconductor laser to be turned on continuously, or intermittently with the high period, a BD is scanned with the laser beam within one scanning period, and the BD generates a BD signal. Based on a generation timing of the BD signal, the image forming apparatus specifies a timing of scanning with the laser beam on the BD within the one scanning period. Based on the specified result, the image forming apparatus causes the semiconductor laser to emit a laser beam. Thereby the laser beam can be entered onto the BD without outputting a laser beam with which a photosensitive member is scanned. The BD signal generated stably allows use of a period of the BD signal to control the rotational speed of the rotary polygon mirror. Then the image forming apparatus starts image formation as the rotational speed of the rotary polygon mirror is stabilized.
In BD search operation, the photosensitive member is scanned with the laser beam before the BD signal is generated. There is an apparatus in which a charge device and a developing device are started up before the BD search. In such an apparatus, a toner image is formed by scanning with a laser beam on a photosensitive member, and toner is consumed. In order to suppress toner consumption, the charge device and the developing device have to be started up after the BD search. In such a case, other devices can be started up only after the BD search is finished. Such a configuration raises a problem in that a long time is required as a startup time of the entire apparatus.
The present invention is made in such circumstances. The present invention is directed to shortening a startup time of the entire image forming apparatus without forming an unnecessary image on a photosensitive member.